The invention relates to optical devices and in particular to devices for wavelength-division multiplexing of optical signals and/or demultiplexing such multiplexed signals. The invention is especially, but not exclusively, applicable to so-called xe2x80x9cfree spacexe2x80x9d multiplexer/demultiplexer devices in which the light beams traversing the device are not guided.
Increasingly, optical communications systems use wavelength division multiplexing (WDM) to increase capacity by transmitting data at several different wavelengths, simultaneously, via the same communications channel, for example via the same optical fiber. Such increased capacity results in lower optical fiber requirements and allows existing systems to be upgraded to handle increases in data traffic.
In order to implement WDM, optical transmission systems require a multiplexer for multiplexing several signals having different wavelengths to form a single WDM signal for transmission, and a demultiplexer for demultiplexing the WDM signal to extract the original signals having different wavelengths. The multiplexer and the demultiplexer may be identical devices, but with light traversing them in opposite directions.
The invention is concerned especially with multiplexers/demultiplexers which use an angularly-dispersive element, such as a diffraction grating. Examples of such devices are disclosed in U.S. Pat. No. 4,622,662 (Laude et al.), U.S. Pat. No. 4,819,224 (Laude et al.), U.S. Pat. No. 4,926,412 (Jannson et al.), and French patent document No. 2,731,573 (Laude). Such devices include a diffraction grating, a collimator lens and an array of optical waveguides, specifically fibers. When the device is used as a demultiplexer, the input WDM light beam is collimated and then separated by the diffraction grating into a plurality of light beams having different wavelengths and the collimator lens focuses each light beam onto a respective one of the output fibers.
In WDM systems, the intensity of the transmitted/received signals may vary as a result of relative shift of the signal wavelength and the gaussian-type shape of the spectral response of the multiplexer/demultiplexer. Another problem due to the shoulders/wings of the gaussian-type spectral response not being steep enough is an associated overlapping between signals in adjacent communications channels resulting in cross-talk, i.e. poor isolation. These problems can be ameliorated by making the spectral response of the multiplexer/demultiplexer relatively flat, so that variations in the signal wavelength are less likely to affect intensity. It is desirable, therefore, to obtain a spectral response which comprises a substantially flat passband with sharply rising edges/steep shoulders. Such flattening could be obtained by making the optical source spatially wider than the output waveguides, but this would increase transmission loss because each waveguide would intercept only a small portion of the associated light beam.
In an article entitled xe2x80x9cAn original low-loss and pass-band flattened SiO2 on Si planar wavelength demultiplexerxe2x80x9d, OFC ""98 Technical Digest, February 1998, page 77, G. H. B. Thompson et al. proposed instead to use two waveguide arrays with equal diffraction efficiency arranged in tandem, with the image of the first array forming the source for the second array. Such a waveguide array multiplexer/demultiplexer would have a flat-topped response and so be less likely to suffer from poor isolation between channels. However, it would still suffer from high losses and require active temperature stabilization.
The spectral response of a wavelength-division multiplexer using a single grating resembles the response shape of coupling loss as a function of offset between two fibers. This is a convolution between two gaussians, i.e. it is itself a gaussian function. The ratio of channel passband width to spacing can be increased, and the spectral response flattened, by using an even number of gratings in order to cancel out the linear dispersion at the output fiber. This can be done with one grating and an array of retroreflectors shifted one relative to another in a plane perpendicular to the grating""s dispersion plane. Such an approach to flattening the response characteristic of a multiplexer/demultiplexer of the xe2x80x9cfree spacexe2x80x9d kind was disclosed by Isao Nishi et al. in a first article entitled xe2x80x9cBroad-Passband-Width Optical Filter for Multi/Demultiplexer using a Diffraction Grating and a Retroreflector Prismxe2x80x9d, Electronics Letters, Vol. 21, No. 10, May 1985, pp. 423 and 424, and in a second article entitled xe2x80x9cBroad Passband Multi/Demultiplexer for Multimode Fibers Using a Diffraction Grating and Retroreflectorsxe2x80x9d, Journal of Lightwave Technology, Vol. LT-5, No. 12, December 1987, pp. 1695-1700. In Nishi et al.""s device, the retroreflectors are placed where usually the output fiber array would be positioned. Input WDM light is diffracted a first time by the diffraction grating, reflected by the retroreflector back to the diffraction grating where it is diffracted a second time, and then focused onto the output fiber array. This arrangement is not entirely satisfactory for several reasons. In particular, passing the light through the diffraction grating twice may increase polarization-dependent loss. Also, although Nishi et al.""s device might be suitable for multimode fiber arrays and relatively broad bandwidths, there is an increasing demand for narrower bandwidths and single mode fiber arrays. The smaller dimensions involved may lead to difficulties in making very small retroreflectors and aligning them precisely. For example, Nishi et al. described a two channel wavelength division multiplexer with 100 nm channel spacing using two retroreflectors, each with base width W=250xcexc, and length L=100xcexc, with a lateral offset of 62xcexc between the retroreflectors. The retroreflectors were separate elements assembled and glued individually. Increasing the number of channels to, say, eight, with a spacing of 1.6 nm, for example, would require retroreflectors with width and length approximately equal to 150xcexc and 50xcexc, respectively, and a shift of about 10xcexc between them. Manufacture of these elements, and their assembly in a WDM device, would pose major technological problems.
An object of the present invention is to ameliorate the disadvantages of the above-described devices.
According to the present invention, there is provided a multiplexer/demultiplexer device for multiplexing a plurality of light beams communicated via a corresponding plurality of ports to form a wavelength-division-multiplexed (WDM) light beam, each of said plurality of light beams comprising a distinct group of wavelengths, the groups having different centre wavelengths, and for demultiplexing such a wavelength-division-multiplexed (WDM) light beam to form the corresponding plurality of light beams. The multiplexer/demultiplexer comprises:
(i) angular dispersion means for dispersing a said WDM light beam incident thereupon along a predetermined optical path into said plurality of light beams, each at a corresponding one of a plurality of dispersion angles in a dispersion plane of the angular dispersion means, or combining a plurality of light beams incident thereupon each at a corresponding one of said plurality of dispersion angles to form a said WDM light beam in said optical path,
(ii) a WDM port disposed in said optical path for communicating said WDM light beam to or from said angular dispersion means;
(iii) a plurality of ports disposed in said dispersion plane and so positioned relative to said angular dispersion means as to define a corresponding plurality of optical paths for communicating said plurality of light beams between said angular dispersion means and respective ones of said plurality of ports; and
(iv) optics means for spatially-modifying said WDM light beam, or said WDM light beam and each of said plurality of light beams, or each of said plurality of light beams, by refocusing light beam components whose wavelengths are at extremes of the range about the centre wavelengths and defocusing light beam components whose wavelengths are closer to the centre wavelengths so as to achieve a spectral intensity across each port that is substantially flat.
The angular dispersion means may comprise a diffraction grating.
In one preferred embodiment of the invention, the angular dispersion means comprises a diffraction grating and a collimating lens disposed between the diffraction grating and the WDM port for collimating light leaving the WDM port before the light is incident upon the diffraction grating, or conversely focusing a light beam leaving the diffraction grating onto the associated port.
The optics means may comprise a plurality of lens elements, such as microlenses, each for spatially-shaping a respective one of said plurality of light beams and associated with a respective one of the plurality of ports.
Alternatively, the optics means may comprise a lens element, such as a microlens, associated with said WDM port for spatially-shaping said WDM light beam.
The optics means may comprise, in combination, such a plurality of lens elements each for spatially-shaping a respective one of said plurality of light beams and such a lens element associated with the WDM port for spatially-shaping the WDM light beam, the total spatial-modification of a particular light beam, by refocusing light beam components whose wavelengths are at extremes of the range about the centre wavelengths and defocusing light beam components whose wavelengths are closer to the centre wavelengths, being provided cumulatively by the lens associated with the WDM port and the respective one of the plurality of lenses.
Each port may be the end of an optical waveguide, for example an optical fiber.
The or each microlens may comprise a cylindrical lens and may be spaced from the associated one of the ports by a distance about equal to the focal length of the microlens.
The numerical aperture of each optical element being approximately equal to the numerical aperture of the associated port, e.g. fiber or other waveguide, minimizes losses as the optical element focuses the light of slightly different wavelengths incident thereupon into the corresponding port/waveguide.
Where a lens element is provided adjacent an input port, a light beam entering the port and passing through the lens element will be preconditioned, i.e. its width reduced as aforesaid, to such an extent that, following dispersion and focusing, each of the output light beams is concentrated into the required groups of wavelengths and numerical aperture.